Jeff Evans
Having a mixer flip the output polarity is not a big deal and you certainly won’t hear it unless there is more than one mixer in your setup and you happen to have say a stereo signal split over the two mixers which I guess we would not do anyway.
 
At one stage I had 3 sound cards in my computer and one of them flipped the polarity.  On the actual software even non symmetrical waveforms were in phase eg on the arrange window but not in their outputs.
 
The non symmetrical waveform test is very interesting and revealing.  Lots of things showed up as polarity flipped.  A standard sinewave is no good because it will look the same everywhere.
 
The biggest thing I got was when I tested ALL of my microphones and found that about 3 or 4 of them were polarity flipped. Had to rewire the XLR connector to put them back the same way as the others.  Not sure what was going on there but one would assume there is a standard.  May have been a wiring error or some just not caring.  Some external hardware will flip the polarity around too.